watchdog: Convert twl4030_wdt to watchdog core

Convert the twl4030_wdt watchdog driver to watchdog core.

While at there use devm_kzalloc and set the default timeout in order to be
able test this driver with a simple shell script.

Signed-off-by: Jarkko Nikula <jarkko.nikula@jollamobile.com>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>